Electro-optical sensor assemblies use optical components to route and focus received radiation onto an electro-optical detector. The detector may be responsive to longwave infrared radiation (approximately 8 to 10 micron wavelengths), midwave infrared radiation (approximately 3 to 5 micron wavelengths), near-infrared radiation (approximately 0.7 to 0.9 micron wavelengths), laser light (such as 1.06 to 1.54 micron wavelengths), or radiation having other wavelengths. Frequently, a sensor assembly will be a multi-spectral device having two or more detectors which are responsive to radiation at respective different wavelengths, and which require separate imaging optics.
The size and weight of electro-optical sensor assemblies has always been a significant design consideration, and the problem is compounded when a sensor assembly includes multiple detectors. In an airborne application, the size of the sensor assembly dictates the size of the required gimbal, which in turn affects the overall system size and weight. Since the sensor assembly and gimbal may both be in the airstream, the size of each can affect the overall aircraft drag. In ground applications, where a head mirror is used for elevation pointing, the number of optical apertures and the size of these apertures dictate the head mirror size. In turn, the head mirror size affects the size and weight of the surrounding armor plate.
Multi-detector sensor assemblies may have other requirements that affect size, weight and/or performance. Imaging optics with long optical paths can result in optical losses, or in other words poor optical transmission. Also, two-dimensional infrared staring arrays with excellent resolution are becoming available, and where one or more of the detectors in a sensor assembly is such a staring array, optical-mechanical image motion stabilization (IMS) may be required in order to permit the overall system to output an image having the high resolution which the staring array itself is capable of producing.
One known approach for reducing the size of a multi-detector sensor assembly is to use a common aperture for two or more of the detectors. One known system uses a completely reflective afocal up front, which transmits all wavelengths. Tilted beam splitters are used after the afocal in order to separate different wavelengths for the respective detectors. A respective different refractive lens system is typically provided after the beam splitters for each detector.
In this known approach, the tilted beam splitters are located in collimated light, where aberrations are not a problems. However, the afocal itself is rather large. Moreover, multiple fields of view are difficult to achieve with a completely reflective afocal. The afocal can be totally bypassed for a second field of view, but if three fields of view are required, the optical system must transmit at all sensor wavelengths. If additional fields of view are made refractive, the materials used may not transmit some wavelengths of interest. If reflective optics are employed, large fields of view are difficult to achieve.
An all-refractive optic approach has been used in a single aperture, multi-spectral sensor assembly, one example of which is U.S. Pat. No. 4,621,888. This assembly can be used to achieve a common aperture for visible wavelengths and for infrared wavelengths up to 12 microns. However, only the front lens is common to both detectors, and the optical paths are relatively long, so that the size of the assembly is relatively large. U.S. Pat. Nos. 4,871,219 and 4,999,005 are other examples of multi-spectral systems having refractive optics which use a common aperture. In these systems, the detectors are limited to detection of just infrared radiation.